The invention relates to a process and device for controlling the drive mechanism of a conveyor system in the form of an escalator or passenger conveyor which can be switched between load and no-load operation. The conveyor system comprises AC line voltage terminals, an electric drive motor, specifically, in the form of an induction motor or synchronous motor, and a frequency converter.
A typical conveyor system for the transportation of passengers in the form of an escalator or passenger conveyor includes a plurality of closely adjacent running plates in the form of a continuous belt which are moved with the aid of the drive motor.
In order to reduce the power consumption and wear and tear on these types of conveyor systems, methods have been developed in which the conveyor movement of such conveyor systems is activated only when conveyance is required, while bringing the system to a stop otherwise. For this purpose, a device signaling the need for conveyance, for instance, in the form of a step mat which is arranged in the direction of conveyance in front of the conveyor system, an on/off photocell control device, or a manually operated switch, is provided, by means of which the presence of the need for conveyance can be determined. If a need for conveyance exists, for instance, because a passenger has stepped onto the step mat, the conveyor system is set in the conveyance motion for a predefined period of time and turned off again if no further need for conveyance has been detected within a predefined time period.
Preventing the abrupt turning on and off of the drive motor during frequent starting and stopping of the conveyor system to avoid peak loading by instead allowing the rotational speed of the drive motor to ramp up and ramp down is known in the prior art from WO 98/18711. Induction motors are predominantly used for these types of conveyor systems. Because the rotational speed of an induction motor, which depends on the frequency of the alternating voltage supplied to it, will be constant when it is directly supplied from an AC line with constant line frequency, an adjustable frequency converter is employed so that the line frequency supplied to it can be converted into a different output frequency.
The cost of a frequency converter which supplies the drive motor of an escalator or passenger conveyor under load is high because the cost of the frequency converter increases in direct proportion to the output power the frequency converter is capable of delivering.
In order to lower acquisition and operating costs, WO 98/18711 stipulates that the conveyor system be driven with full conveyance speed only under load but that in stand-by or in no-load operation, when no conveyance need exists, it is only driven at reduced, no-load speed, and that the drive motor will be supplied by the frequency converter only under no-load operation and changeover processes, while, however, under load, it will be supplied directly by the AC power line. This makes it possible to design the frequency converter with a much lower maximum power rating, which results in considerable cost savings compared to a frequency converter whose maximum power rating is matched to the load of the conveyor system. Then, if after carrying out a conveyance task, no further need for conveyance is indicated, the conveyor system known in the prior art from WO 98/18711 initially changes over to no-load operation and is not switched to stop until no new need for conveyance is indicated for a predefined time period after its changeover to no-load operation.
As a result of the measures mentioned, a considerable reduction of peak loads and abrupt changes in the speed of the conveyor systems is realized. However, while the drive motor alternates between being supplied power by the AC line and by the frequency converter, high transient currents may still occur; to be more specific, this is due to deviations between the line frequency and the output frequency of the frequency converter and their phase angles at the time of a changeover between power line and the frequency converter feeding of the drive motor and because of the induced voltage of the drive motor which can lead to overloading the frequency converter and to abrupt changes in the movement of the conveyor system.
Such manifestations have been overcome with a process which was disclosed in the subsequently published previous German Patent Application 199 60 491.6 of the applicant and in which the line voltage and the frequency converter output voltage are compared with respect to frequency and phase angle, and the frequency converter is controlled to an output frequency that exhibits a predefined frequency spacing from the line frequency. If a need for a changeover of the conveyor system from load to no-load operation or vice versa has been signaled by means of a conveyor signaling device, a changeover control signal which triggers the changeover of the power supplied to the drive motor by the frequency converter feeding and the power line is generated at that point in time after the need to switch operating modes has been signaled, at which point the output frequency of the frequency converter both exhibits the predefined frequency spacing with respect to the line frequency and a predefined phase spacing has been achieved between the output frequency of the frequency converter and the line frequency. By not emitting the changeover control signal at time when the output frequency of the frequency converter matches the line frequency, with respect to both frequency and phase, but instead, xe2x80x9cwith foresight,xe2x80x9d emitting it at time when the output frequency of the frequency converter exhibits the predefined frequency spacing with respect to the line frequency and the predefined phase spacing between the output frequency of the frequency converter and the line frequency have been reached, consideration is given to the fact that switching devices, usually contactors, which are employed for the changeover between no-load and load operations, on the one hand, work without time lags and that, on the other hand, a zero-current period is required between the break of one contactor and the make of the other contactor, in order to avoid short-circuiting the network via the frequency converter. Between the emission of a changeover control signal and the break of the previously leading contactor, and, finally, the make of the other contactor, a certain inherent response time delay exists which depends on the specific components of the specific conveyor system and is given consideration through the aforementioned frequency spacing and the aforementioned phase spacing.
The process described in the German Patent Application 199 60 491.6 has proven to be successful. There are cases, however, in which fewer control expenditures are desired and this is to be realized with the present invention.
The present invention provides a process for controlling the drive mechanism of a conveyor system which can be switched between load and no-load operation, specifically, in the form of an escalator or passenger conveyor, exhibiting a drive motor and a variable frequency converter which, at least, is controllable with respect to the frequency of its output voltage, wherein, under load, the drive motor is supplied by the line voltage with an essentially constant line frequency, and at no-load, it is supplied with the output voltage of the variable frequency converter, the output variable frequency of the frequency converter, prior to a changeover from no-load to load operation or vice versa, is essentially brought to the line frequency with accurate phase by means of a PLL device, and this changeover is achieved as soon as this phase agreement occurs.
On the other hand, the invention provides electronic control device for controlling the drive mechanism of a conveyor system, specifically, in the form of an escalator or passenger conveyor, which can be switched between load and no-load operation and which comprises an AC line voltage terminal for supplying line voltage with an essentially constant line frequency and a drive motor, comprising a variable frequency converter, which at least is controllable with regard to its output voltage frequency, a controllable changeover device with a load operation switching state in which the drive motor is coupled to the AC line voltage, and a no-load switching state in which the drive motor is coupled to the variable frequency converter, in such a way that under load, the drive motor is supplied the line voltage with an essentially constant line frequency and at no-load is supplied the output voltage of the variable frequency converter, and a PLL device, by means of which, prior to a changeover from no-load to load operation or vice versa, the output frequency of the variable frequency converter can essentially be matched to the line frequency with phase accuracy, where the changeover of the changeover device is controllable as a function of whether such phase matching is achieved.
In one embodiment of the invention, the line frequency is fed to the PLL device on the input side as the setpoint phase and the output frequency of the variable frequency converter is fed as the actual phase. In this manner, the output frequency of the variable frequency converter is changed until the output frequency of the PLL device and, hence, of the variable frequency converter, locks to the line frequency.
In one embodiment of the invention, in the context of a changeover from no-load to load operation, a ramped rise of the output frequency of the variable frequency converter is controlled first before the output frequency of the variable frequency converter is brought to the line frequency by means of the PLL device and a changeover is carried out from variable frequency converter supply to the line supply. In the same manner, a ramped decrease in the output frequency of the variable frequency converter can be controlled during a changeover from load to no-load operation after a changeover has been carried out from the line supply to the variable frequency converter supply. Thus, the changes in the rate of movement of the conveyor system are made more gradual, so that changes from both no-load to load operation and from load to no-load operation can be made unabruptly.
In one embodiment of the invention, the changeover from no-load to load operation occurs by means of a changeover device which exhibits a first controllable switching device which connects the drive motor to the AC line voltage, and a second controllable switching device which connects the drive motor to the variable frequency converter, wherein only one of the two switching devices can be electrically connected and the electrical connection of the respectively disconnected switching device is only possible after a predefined zero-current period after the switching device which has been electrically connected to that point has been disconnected. This takes into consideration that the contactors which are usually employed for such switching devices do not work without a time lag and ensures that a simultaneous connection of both switching devices will not occur, which could result in a damaging short circuit of the network via the variable frequency converter.
During the zero-current period, the drive motor remains without power supply, which leads to a decrease in the rotational speed of the drive motor during the zero-current period due to the inherent friction of the conveyor system, which, in turn, results in a reduction in the magnitude and frequency of the motor terminal voltage.
In order to avoid impairment of the smooth changeover between no-load and load operation due to these occurrences during these zero-current periods, a third controllable switching is provided in one embodiment of the invention, by means of which the setpoint phase input terminal of the PLL device is connected to the AC line voltage in a first switching state and to a motor terminal at which the motor terminal voltage can be picked up in a second switching state, wherein the third switching device is controlled in such a way that during the zero-current period, the setpoint phase input terminal is coupled to the motor terminal and otherwise is coupled to the AC line voltage. Thus, the output frequency of the variable frequency converter can be brought to a frequency during the zero-current period of the drive motor that corresponds to the decrease in the rotational speed of the drive motor during the zero-current period.
This measure has the effect that, during a changeover from load to no-load operation, i.e., during a changeover from line supply to the variable frequency converter supply at the time point that the motor is connected to the output of the variable frequency converter, the output frequency of the variable frequency converter is frequency- and phase-matched to the rotational speed and the motor rotation position of the drive motor.
Because, during the zero-current period, the rotational speed of the drive motor decreases, one embodiment of the invention provides that, prior to the changeover process, the variable frequency converter regulates the drive motor to a rotational speed when the changeover from no-load to load operation occurs, which exceeds the motor rotational speed which corresponds to the line frequency, by the amount by which the motor rotational speed decreases during the zero-current period. The amount by which the motor rotation speed decreases during the zero-current period can be determined for the respective conveyor system, for instance, by taking measurements, and can be taken into consideration in the design of the control circuit of the variable frequency converter.
Because not only the motor rotation speed but also the motor terminal voltage decreases during the zero-current period, one embodiment of the invention provides that the output voltage of the variable frequency converter be adjustable, that the motor terminal voltage be measured at least during the zero-current period, and that the output voltage of the variable frequency converter be matched to the motor terminal voltage during the zero-current period. This also facilitates a smooth transition during the changeover from the line supply to the variable frequency converter supply of the drive motor.
Because, normally, the motor data and the zero-current period for any specific conveyor system are known values, the decrease in the motor terminal voltage which occurs during the zero-current period can be determined from said data and used. In this case, a measuring device for the motor voltage is not required.
In their output stage, standard variable frequency converters contain bridge circuits with electronic switches that are triggered by switch control pulses, the frequency of which determines the output frequency of the variable frequency converter. In one embodiment of the invention, the control of the value of the variable frequency converter output voltage, which was discussed previously, is brought about by pulse-width modulation of the switch control pulses.
In one embodiment of the invention, the PLL device, in the standard series connection, comprises a phase detector, a loop filter, and a voltage-controlled oscillator, wherein the line voltage or, during the zero-current period, the motor terminal voltage is supplied to the phase detector as the setpoint signal and the voltage of the variable frequency converter is supplied as the actual signal.
In one embodiment of the invention, the variable frequency converter and at least the voltage-controlled oscillator of the PLL device are integrated in a variable frequency converter circuit arrangement. The phase detector and/or the loop filter may be realized as hardware components or in software by programming the variable frequency converter circuit arrangement. In the latter case, the actual value can be sensed from OVF 42 and processed directly in the software module for the PLL device as the actual value.
In the event that the variable frequency converter is realized through a programmable circuit arrangement and that the PLL function is brought about by programming such a variable frequency converter, a control device in accordance with the invention can be produced with a particularly low expenditure and thus at particularly low cost.